The present invention relates to a method for fabricating a photomask pattern by which a semiconductor device such as an integrated circuit (IC) or a large scale integrated circuit (LSI) is fabricated.
Before disclosing the present invention, a feature and a fabricating process of the photomask will be explained as background of the invention, and problems of a prior art method for the photomask pattern fabrication will be discussed.
The photomask is made of a base plate of silica glass on which a metal layer and a photoresist layer are sputtered and coated as shown in FIG. 1. The figure illustrates a cut view of the photomask 200; a metal layer 2 of chromium is sputtered on a glass plate 1 and photoresist layer 3 is coated on the metal layer 2. The photoresist pattern is fabricated by processes of exposing an optical image on the photomask, developing and etching it. FIG. 2 illustrates a plan view of the photomask 200 on which a photomask pattern 210 is printed. The photomask pattern 210 consists of plurality of unit patterns 201 separated by scribe lines 220. Each unit pattern 201 has the same shape and size, and corresponds to a pattern for a semiconductor die; this photomask pattern 210 is for a mass production of the semiconductor devices.
FIG. 3 illustrates a printing process for the photomask pattern. In the figure, each unit pattern 201 is printed by exposing an optical image of a reticle pattern 101 on a reticle 100 by means of an optical system 400. The reticle pattern 101 is a mother pattern for the semi-conductor die and the size thereof is as large as 10 times that of the exposed unit pattern 201 to perform high accurate printing; accordingly the optical system 400 has a lens 401 having a reduction factor of one-tenth. Each unit pattern 201 is printed in succession by shifting the photomask 200 toward X and Y directions with respect to the fixed optical system 400, which is called a "step-and-repeat printing". An apparatus for the step-and-repeat printing will be explained in more detail with respect to FIG. 4.
FIG. 4 shows a block diagram of an example of the step-and-repeat printing apparatus. In the figure, the photomask 200 is mounted on a stage 500 which is shifted toward X and Y directions as controlled by a stage controller 800. The sequence of the step-and-repeat printing is previously determined and is stored in a memory 600. The data in the memory 600 are read out and amplified by a amplifier 700 and go to the stage controller 800. The data fed to the stage controller 800 controls the stage movement and also controls a light controller 900 to control the light source 402 in the optical system 400 as shown in FIG. 3, synchronization with the step-andrepeat movement of the stage 500.
FIG. 5 is a schematic diagram illustrating the prior art sequence of the step-and-repeat printing, showing an arrangement of unit patterns in a matrix having rows and columns on a photomask pattern area of the photomask. The step-and-repeat printing is performed so that the exposure of the unit pattern is advanced from a leftmost matrix element 11 to a matrix element 12, 13,--sequentially in a direction to the right along the first row as shown by the arrow A, and when the exposure of the first row is finished, the exposed is shifted down to the rightmost matrix element 16 in the second row and advanced toward the left as shown by an arrow B; thus, the exposure is advanced in the matrix in zigzag pattern as shown by arrows A, B, C,--, to repeat a step-and-repeat exposure with an equal pitch "P" in the row and column directions.
As the semiconductor circuit is complicated, a plurality of photomasks is required to fabricate the semiconductor device from a wafer, and the position of the unit patterns on each photomask must coincide with each other when they are overlapped to make up the pattern of the IC device. Therefore, the unit patterns on the photomask, and especially their positions, must be carefully inspected after printing. The inspection is performed by observing vernier patterns printed together with each of the unit patterns on the photomask through a microscope. FIG. 6 shows an example of a vernier pattern called a cross-shaped vernier pattern which has four branches top, bottom, left and right. The respective numbers of teeth of the top and bottom branches, and those of the left and right branches, are different by one tooth, which combination provides a vernier. In the reticle pattern, mother patterns of the vernier patterns are provided at the respective four corners, so that the numbers of the teeth at each of the top and bottom and left and right branches become different in the overlapping cross-shaped vernier patterns. The vernier patterns of adjacent unit patterns are overlapped with each other when they are printed on the photomask. FIG. 7 shows this situation; for example, a pattern 230 is formed by overlapping the vernier patterns of the unit patterns 202, 203, 204, and 205. Accordingly, the shear of unit patterns which are printed adjacent each other can be measured by observing the pattern 230, and the kind of this pattern 230 will be called a "overlapped vernier pattern" hereinafter. As the number of the unit patterns on the photomask is as many as several thousands in some cases, it is actually impossible to measure all the overlapped vernier patterns. However, it is well known from the art of inspection that several points are enough for the inspection, for example, four points each placed near the middle part of four respective sides of the photomask pattern, for instance, as shown by points a, b, c, and d in FIG. 5, are enough to inspect the positions of unit patterns on the photomask pattern.
In the prior art method however, there have been problems as follows.
(1) The printed shear in the exposing process is accumulated as the step is advanced along the row of unit patterns as shown by the arrows A, B, or C in FIG. 5.
(2) The shear accumulation occurs by electrical and mechanical causes which depend on a feature of the printing apparatus, and it unpredictably happens in some interval as shown by the coordinates in FIG. 8. In the coordinates, the abscissa t is a time of a photomask pattern exposure, the ordinate s is a maximum amount of the shear measured in absolute value obtained by measuring the shear in the photomask pattern, a dotted line R indicates an allowable maximum value of the shear from the ideal position of the unit pattern, and the curve M shows an actual amount of the shear which occurred. In the figure, t.sub.1, t.sub.2,--, t.sub.9 show the time, and if it is assumed that the exposure of the patterns has been done in the respective time intervals from t.sub.1 to t.sub.2, t.sub.2 to t.sub.3,--, or t.sub.8 to t.sub.9, the exposure which has been performed in the interval from t.sub.1 to t.sub.2, t.sub.2 to t.sub.3, t.sub.6 to t.sub.7, t.sub.7 to t.sub.8, or t.sub.8 to t.sub.9 ("o" is respectively indicated) results in a good printing, but the exposure which has been performed in the time interval from t.sub.3 to t.sub.4, t.sub.4 to t.sub.5, or t.sub.5 to t.sub.6 ("x" is respectively indicated) results in a bad printing. The time interval for the exposure is a length of several tens of minutes, and the time interval in which the shear begins to exceed the allowable amount R occurs frequently in a period of several hours.
(3) It is impossible to detect the printing shear which exceeds the allowable value of R by observing and measuring the vernier patterns, because each unit pattern respectively has almost the equal amount of the accumulated shear, so that the zero deviation of the vernier does not show the absolute zero deviation.
(4) The absolute amount of the shear can be measured by a laser measuring system. However, a large amount of time is required for the measurement, such as for optical setting. Therefore, the laser measurement is usually used in a laboratory, and the vernier measurement is still used for actual fabrication of the photomask pattern.